Heretofore, there have been known various constitutions of clay kneaders for use in kneading and homogenizing a clay including, for example, a ceramic material by the rotation of a screw in a drum to prepare a columnar article (e.g., see Patent Documents 1 and 2).
FIG. 2 is a schematic constitution diagram showing one example of a conventional clay kneader. As shown in FIG. 2, the conventional clay kneader has a structure in which a drum 2 is provided with a screw 4 therein. The screw 4 disposed in the drum 2 imparts a large shear force to the clay which flows from a supply opening 19 into the drum 2, to knead and homogenize the clay. Then, the clay kneaded and homogenized in the drum 2 is extruded to manufacture a columnar article 15.
Moreover, as a method of manufacturing a formed ceramic honeycomb article, a method is known which blends a ceramic material, water, a binder and the like, then prepares a kneaded clay as a large clay-like lump by a kneader, homogenizes the clay by deaeration and kneading by use of a clay kneader, simultaneously extrudes the clay to prepare the columnar article, and extrusion-forms (by a plunger type forming machine) this columnar article to manufacture the formed ceramic honeycomb article.
In this case, the columnar article is prepared by the clay kneader, and a flow pattern of a clay density is sometimes partially formed by the rotation of the screw of the clay kneader. Moreover, this flow pattern sometimes noticeably curves in an extrusion direction, depending on the structure of the distal end of the clay kneader. When a ceramic honeycomb structure is extrusion-formed by using such a columnar article, various formed article defects are generated. Examples of the defects include a screw-trace-like internal defect. Moreover, this defect is remarkably generated, when a larger amount of a pore former is contained.
Especially in recent years, the thinning of partition walls in the ceramic honeycomb structure has further progressed. To prepare the ceramic columnar article for use in the formed article having these thin walls, uniform quality is remarkably intensely demanded, and manufacturing steps of the columnar article need to be contrived so as to satisfy strict quality control requirements.    Patent Document 1: JP-A-9-94818    Patent Document 2: JP-A-10-100131
When the ceramic honeycomb structure or the like is extrusion-formed by using the columnar article obtained by the conventional clay kneader having the structure shown in FIG. 2, the flow pattern of clay density is generated in the columnar article with the rotation of the screw in the clay kneader. When this flow pattern noticeably curves in the extrusion direction and the honeycomb structure is formed by using this columnar article, a forming failure is sometimes generated.
FIG. 3 is a schematic explanatory view showing one example of a conventional clay kneader 30. In the screw structure of the conventional clay kneader shown in FIG. 3, an impulsive force is imparted to a clay while kneading the clay in a pressure raising section 10, to extrusion-form the columnar article through an extrusion section 12. At this time, the inner diameters of the pressure raising section 10 and the extrusion section 12 hardly change, and hence the flow pattern hardly curves in an extrusion direction 41. If the flow pattern curves, such curvature has little influence, and hence the forming failure is hardly seen in a case where the honeycomb structure is formed by using this columnar article.
However, as shown in the schematic explanatory view of FIG. 4 showing another example of the conventional clay kneader 30 and the schematic sectional view of FIG. 6 showing a distal end structure of the conventional clay kneader, in a case where an extrusion opening 10 is provided in such a region that the inner diameter of the extrusion opening is smaller than that of a pressure raising section 10 and as one example, the inner diameter is decreased from 45 cm to 30 cm with a decrease width of 15 cm, that is, a decrease ratio of about 67%, the flow pattern noticeably curves on the extrusion opening 41 side in a portion where the inner diameter of an extrusion section 12 decreases in the extrusion section 12. This conventional clay kneader 30 is sometimes used for a purpose of increasing the amount of the clay to be treated per unit time without changing the diameter of the columnar article. This is because as the inner diameter of the pressure raising section 10 is large, capability for treating the clay can be improved.
FIG. 8 is a schematic sectional view showing the side surface of the columnar article 15 obtained by using the conventional clay kneader 30 of FIG. 4. As shown in this drawing, flow patterns 17 are formed owing to a density difference in the clay so that the patterns are symmetric with respect to a rotary center line 16 of the columnar article 15 and noticeably curve in the extrusion direction 41. When the honeycomb structure or the like is formed by using the columnar article having these flow patterns 17, the above problems occur.